1. Field of the Invention
The present invention relates to a binder for a toner which is used in a dry developer used for an image forming process such as electrophotography, electrostatic recording or magnetic recording.
2. Related Background Art
Methods have been conventionally known as electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member, utilizing a photoconductive material and according to various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. In the case when the process comprises a toner-image transfer step, the process is usually provided with the step of removing the toner remaining on a photosensitive member.
As developing processes in which an electrostatic latent image is formed into a visible image by the use of a toner, known methods include the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, and the method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258.
As toners used in these development processes, fine powder obtained by dispersing a dye and/or pigment in a natural or synthetic resin has been hitherto used. For example, colored resin particles formed by finely grinding a binder resin such as polystyrene comprising a colorant dispersed therein, to have a size of about 1 to 30 .mu. are used as the toner. A toner incorporated with magnetic material particles such as magnetite or ferrite is also used as the magnetic toner. In a system in which a two-component type developer is used, the toner is usually used by mixture with carrier particles such as glass beads, ion powder and ferrite particles.
Nowadays, such recording processes have been widely utilized not only in commonly available copying machines, but also in output means of computers or facsimile machines or for the printing of microfilms. Accordingly, a higher performance has become required, and the above recording processes have now been required to simultaneously achieve the improvements in performance such that an apparatus is made small-sized, lightweight, low-energy, high-speed, maintenance-free, and personal. In order to meet these requirements, the needs on toners have become severer in various aspects. For example, when the copying machines or printers are made small-sized, heat sources such as heat-fixing assemblies and exposure lamps are squeezed into a narrow space, so that the temperature inside the machine tends to become higher. Hence, toners must be made to have an improved blocking resistance. In order to make the copying machines or printers lightweight, a fixing roller is so designed as to be more thin-walled and slender, and a cleaning mechanism for a heat-fixing roller or a cleaning mechanism for a photosensitive member tends to be more simple and lightweight. Thus, there is a tendency that the machine is provided with no applicator used for applying an anti-offset oil to a fixing unit. This makes it necessary to improve fixing properties of toners, offset resistance thereof, and cleaning resistance of photosensitive members. In order to make the copying machines, etc. more small-energy or to make development more high-speed, the fixing properties of toners must be improved as a matter of course. In order to make the copying machines or printers more personal, the reliability must be improved, and it becomes important to cause no paper jam. The paper jam may commonly often occur when a transfer sheet (copy paper) winds around a roller, and thus it becomes necessary for toners to have the properties of suppressing the winding of paper around a fixing roller. However, as shown in FIG. 1, the performances required when a toner is prepared and the properties of a toner itself often conflict with each other.
The needs on toners are severe as will be seen from the above instances, and it is difficult to meet the requirements unless these performance and properties are simultaneously improved. However, it would be nonsense if the achievement of these improvements results in lowering of development performance such as image quality and durability of toners, and production efficiency of toners.
These greatly depend on the performance of the binder resin used in toners. It has been proposed to improve the characteristics of a toner by the use of a release agent, a plasticizer or other additives. Use of these, however, is a supplementary means.
Various methods have been proposed for the improvement of binder resins used in toners.
For example, Japanese Patent Application Laid-open No. 56-158340 proposes a toner containing a binder resin comprised of a low-molecular weight polymer and a high-molecular weight polymer. In reality, it is difficult for this binder resin to be incorporated with a cross-linking component. Hence, in order to improve the offset resistance of toners, it is necessary to make the molecular weight of the high-molecular weight polymer larger or to increase the proportion of the high-molecular weight polymer. This tends to cause extreme lowering of the grindability of a toner.
In relation to a toner containing a binder resin comprised of a blend of a low-molecular weight polymer with a cross-linked polymer, Japanese Patent Application Laid-open No. 58-86558 also proposes a toner comprising a low-molecular weight polymer and an insoluble infusible high-molecular weight polymer as main resin components. According to this technique, the fixing properties and grindability are presumed to be improved. However, it is difficult to satisfy in a high performance both the offset resistance and the grindability at the time of the manufacture of toners, because the weight average molecular weight/number average molecular weight (Mw/Mn) of the low-molecular weight polymer is as small as not more than 3.5 and the content of the insoluble infusible high-molecular weight polymer is as large as from 40 to 90 wt. %. Thus, it is very difficult from a practical viewpoint to give a toner well satisfying the fixing performance and the offset resistance unless a fixing machine is provided with an apparatus for feeding an anti-offset fluid. Moreover, the toner must be heat-kneaded at a temperature far higher than that in usual instances or heat-kneaded at a high shear, because the insoluble infusible high-molecular weight polymer used in a larger amount tends to have a very high melt viscosity as a result of the heat-kneading carried out when the toner is prepared. As a result, in the instance where it is kneaded at a high temperature, the toner characteristics tend to be lowered because of thermal decomposition of other additives. In the instance where it is kneaded at a high shear, the molecules of the binder resin may be excessively cut. Thus, there is the problem that the desired offset resistance can be achieved with difficulty.
Japanese Patent Application Laid-open No. 60-166958 proposes a toner comprising a resin composition obtained by polymerizing monomers in the presence of a low-molecular weight poly(.alpha.-methylstyrene) having a number average molecular weight of from 500 to 1,500.
In particular, this publication discloses that the number average molecular weight (Mn) may preferably range from 9,000 to 30,000. With an increase in Mn for the purpose of improving offset resistance of a toner, the fixing properties of the toner and the grindability at the time the toner is prepared may become more questionable from a practical viewpoint. Hence it is difficult to satisfy in a high performance the offset resistance and the grindability. Thus, the toner having a poor grindability at the time the toner is prepared brings about lowering of production efficiency, and also coarse toner particles tend to be included into the toner, undesirably resulting in black spots around a toner image.
Japanese Patent Application Laid-open No. 56-16144 proposes a toner containing a binder resin component having at least one maximum value in each region of a molecular weight of from 10.sup.3 to 8.times.10.sup.4 and a molecular weight of from 10.sup.5 to 2.times.10.sup.6, in the molecular weight distribution measured by gel permeation chromatography (GPC). This toner can give a superiority in the grindability, offset resistance, fixing properties, anti-filming or anti-fusing to a photosensitive member, image quality, etc. The toner, however, is sought to be further improved in the offset resistance and fixing properties. In particular, it is sought to more improve the fixing properties while maintaining or improving other various performances, to cope with the recent severe demands.
Japanese Patent Application Laid-open No. 63-223014 proposes a binder resin that can solve such a problem. At present, however, it is required to achieve a much higher durability and reliability.
Binder resins greatly influence the developability of toners, and it has been sought to provide such a binder resin that makes stable the quantity of triboelectricity of a toner.
In addition, even if a toner has an excellent offset resistance, it may occur that a fixing roller is contaminated as the toner is used, bringing about a deterioration of offset resistance.
In a heat-pressure fixing method, Japanese Patent Application Laid-open No. 55-134861 proposes to use a binder resin containing an acid component for the purpose of improving fixing performance.
The toner making use of such a binder resin, however, tends to cause a charge insufficiency under conditions of a high humidity and a charge excess under conditions of a low humidity and also tends to be affected by environmental variations, so that fog may be caused and image density may be lowered to make it impossible to obtain a sufficient developability.
On the other hand, an acid anhydride has the action of improving chargeability. Japanese Patent Applications Laid-open No. 59-139053 and No. 62-280758 propose toners employing a resin containing such an acid anhydride. These employ a method in which a polymer having a large number of acid anhydride groups is diluted in a binder resin. In these methods, the resin containing acid anhydride groups must be uniformly dispersed in a binder resin. Unless it is dispersed in a good state, toner particles may be non-uniformly charged, so that fog tends to be caused and the developing performance of a toner may be adversely affected. In these methods, the negative chargeability is so strong that the methods are not preferable for positively chargeable toners.
Japanese Patent Applications Laid-open No. 61-123856 and No. 61-123857 propose toners in which acid anhydride groups are dispersed and diluted by their copolymerization to polymer chains in a binder resin so that the problem of dispersion can be eliminated and a uniform chargeability can be obtained.
Such toners, however, may bring about a charge excess, often causing fog or a lowering of density, when applied in a high-speed copying machine or high-speed printer under conditions of a low humidity. This is due to the fact that the acid anhydride groups in the binder resin used in these toners, though uniformly dispersed, are large in their quantities.